A system for monitoring a machine tool is described, for example, in U.S. Pat. No. 5,822,212. The load of a spindle that is driving a processing tool is monitored. This load is ascertained on the basis of the time characteristic of the current by which the spindle is driven. For the monitoring, values for the spindle load are first ascertained on the basis of testing operations and used as reference values during later processing operations. If the spindle load during these later processing operations deviates from the reference value by more than a still tolerable amount, an alarm will be generated. Then, for example, an interruption of the processing operation may be triggered. Such excessive deviations may be produced by a dull or also a broken tool so that such monitoring is able to prevent more serious damage to a tool.
However, such monitoring devices are frequently also triggering when the triggering would actually not be required. This is frequently the case when processing particularly hard metals such as nickel-chromium alloys known under the trade name of INCONEL. Such alloys can be machined only at particularly low rotational speeds of the tool. Here, conventional monitoring for a lower limit value for the rotational speed quickly leads to a switch-off of the machine tool in case of an overload. Other parameters suitable for the monitoring, e.g., the spindle load, are heavily superposed by the tooth meshing frequency of the tool, which may likewise lead to unnecessary shut-offs.
German Published Patent Application No. 10 2013 202 408 describes dampening chatter vibrations during the milling operation by the action of a filter in the control loop of the drives of a machine tool, the maximum damping of the filter being set to a frequency that takes the instantaneous tooth meshing frequency into account. However, this filter is not suitable for monitoring the milling operation.